1. Field of the Invention
The present invention relates to a process for preparation of N-substituted maleimides. The N-substituted maleimides are useful compounds as agent for modifying heat resistance of resins such as ABS, MMA and PVC, and as intermediate of pharmaceuticals and pesticides.
2. Description of Prior Art
Various processes for preparation of maleimides have been studied. Among them, the most general process is one described in U.S. Pat. No. 2,444,536 in which a maleimide is produced by dehydration of a maleamic acid with a dehydration agent such as acetic anhydride. In detail, maleic anhydride is reacted with an amine compound and the resulting maleamic acid is dehydrated in the presence of acetic anhydride and sodium acetate to perform an imidation reaction and obtain the maleimide.
However, this process requires one or more equivalents of expensive acetic anhydride based on the maleamic acid in the imidation reation. Further, since a large amount of water is used for separation and recovery of the maleimide produced in the reaction, a large quantity of waste water containing acetic acid is produced and a considerable cost is necessitated to treat this waste water. For these reasons, this process is not suitable to utilize in industrial production.
Japanese Patent Disclosure (Kokai) No.53-68770 discloses a process in which maleic anhydride is reacted with an amine compound in an organic solvent, and the resulting maleamic acid is dehydrated in the presence of a nonproton polar solvent and an acid catalyst without further separation from a reaction mixture to obtain a maleimide. This process, however, has the following disadvantages. Since a large amount of the expensive and toxic nonproton polar solvent such as dimethylformamide has to be used, a cost for preparation of the maleimide is relatively high. The nonproton polar solvent is deteriorated by the action of acid catalyst and losses of the nonproton polar solvent are significant. Since the boiling point of the nonproton polar solvent such as dimethylformamide is relatively high, removal of the solvent from the final product containing the maleimide is quite difficult.
Japanese Patent Publication (Kokoku) No.51-40078 discloses the process in which solvents having 80.degree. C. or more of boiling point are used and an N-substituted maleamic acid is dehydrated by heating in the presence of an acid catalyst such as chlorosulfonic acid and removing water generated in the dehydration together with the solvent by azeotropy to obtain a maleimide. This process is excellent because a large amount of expensive dehydration agent such as acetic anhydride is not required, and separation and recovery of the maleimide are easy.
All of the above-mentioned precesses are directed to a processes for preparation of N-substituted maleimides by dehydration of maleamic acids.
Aside from these processes, Japanese patent Disclosure (Kokai) No.62-215563 discloses the process in which a maleic acid monoester is reacted with an isocyanate compound to obtain a maleamic acid monoester and then this monoester is heated to eliminate an alcohol and obtain an N-substituted maleimide. This process, however, requires high degrees of vacuum in the elimination of alcohol and uses the isocyanate compound which is relatively expensive. For these reasons, this process is not suitable to utilize in industrial production.
Among N-substituted maleimides, cycloalkyl substituted and alkyl substituted ones are noted because these maleimides are colorless and can improve heat resistance of colorless resins such as MMA and PVC without coloring them.
The inventors of this invention tried to apply the process described in Japanese Patent Publication (Kokoku) No. 51-40078 mentioned above to the cycloalkyl substituted maleimides. But the yields of these maleimides are extremely lower than that of the maleimides substituted with an aromatic group.
An object of the present invention is to provide a process for preparation of maleimides including cycloalkyl substituted ones in good yield, irrespective of N-substituents.
Another object of the present invention is to provide a process for preparation of maleimides in high selectivity.